Method of spotfastening thin sheets



5. P. JENKINS METHOD OF SP OTFASTENING THIN SHEETS Dec. 9, 1952 2 Sl-IEETS-SHEET 1 Filed April 11. 1947 INVENTOR. 627M 2 (few/(ms 5. P. JENKINS METHOD OF SPOTFASTENING THIN SHEETS Dec.-. 9, 1952 2 smears-swam 2 Filed April 11, 1947 INVENTOR. J'En/ fils/vk/ms v41 me Patented Dec. 9,

" METHOD-F SPOTFASTENING THIN SHEETS S eth;;P.;Jenkins, Los Angeles, Calif., ,assignor to Northrop Aircraft, Inc.,

Hawthorne; Calif a .corporationof California Application April 11, 1947, Serial .No. .7.40-,7.99

--5.Clain1s. (01. 29-1 482) My invention relates: toispotfasteningand more particularly to a means iandmethodof spotfastening thingauge aluminum .alloy heet I to stringers without warping, ,buckling, or canning resulting in the sheet.

In the manufacture-of. aircraft,;.stringer.- toskin fabrication is ahighlydesirabletype ofconstruction but, because of lack of fabrication techniques, a much thickerskinhas oftenbeen used .than required from .a ,rstrength point of view. Thin skins or :sheets .of.aluminum,alloy when spotfastened to rigid-stringers, using .011 tomary methods of welding cor riveting-for.v example, cannot be kept fiatduring attachment as buckling, canning, or warping take place to such an extent that the resultant reinforced sheet cannot be used.

It is known that aluminum-alloy sheets can be spotwelded to stringers -by deforming both stringer and sheet within the elastic limit -of both, priorto spotweldingspotwelding, and then permitting the stringers and the attached skin to return to the original shape. Such methods, however, are subject to several disadvantages because, for example, -when -very thin sheets are tobe fastened to stiff orshortstringers sufficient deformation'of the stiffeners cannot be obtained to stress the skin enough to avoidbuckling. Again, -many stringers are. sotconstructed that 'deformationof any type mustube avoided, thus preventing the desired skin stress. Also many times thin-skins have-to Joeiblind fastened to frames that are in-nowway adapted to abet-deformed.

The present invention does .not have theabove disadvantages, as only thenskin need .be prestressed. In broad terms, theapresent invention comprises -pre-stressing sa-athin sheet .of metal within the elastic limit thereof,spotfasteningunstressed rigid stringers orframes thereto .and then permitting the skin to return to itsioriginal unstressed condition again without stressing the stringers or frame girders. This prestressing of the skin can be accomplished in several ways as, for example, by supporting the skin so that it is corrugated or curved into one or a series of column sections preferably parallel to the spotfastoning line, or by contouring the skin in a single concave or convex column section preferably parallel to the spotfastening line. In either case, spotfastening of sheets as thin as .016 gauge to rigid and stiff stringers by either spotwelding or riveting has been found highly satisfactory, and all normal residual stresses introduced by the spotfastening operation have been found to be equalized in the finished product.

Th veniionwi -ll 1mm fully u d r b re enc t eth d awin i Which= Figure 1 is a perspective vicwof one means of prestressing a. skin p rior to vspotw'elding -to par- .a trin er Figure 2 is a crossse'tional view takenas .indicated by line 2- -2in Eigure l EigureBisape pectivevie 0f anotherimeans ,of prestressing a skin; for thesamepurpose.

, Figures is; a perspective view of .anothermeans for'blind riveting -a. thin skin to .a ;rigid internal fr m Weld- 1 s eche qea la-fl a m g an Sheet ;l of lQlGgauge (24 STALXfor. example, is placed 9 73 I I Earallel; rows; of holes; 3; r are bord in the fixture I 2- where tack welds. are ,to bel made to parallel .str-ingers .1;, in-this case of .2 section, and, for 2O 4 are placed over; the sheet. l (and. held down by fastening means to the fixture" 2, in this .case

e12 or may .j lywood; for ,example.

example, of -.06fl gauge 24-STAL). The stringers wood screws 5 preferablyi atithe ends ofthe stringers. Before screws; are tightened, however,.sl:1irns 6 are p ositionedbetween the sheet 1 and th e fiXture 2. .The screws .are thentightened to stress the sheet l between thestringers and along linesparalleltofthe spotweld center .lines.

Ifhe use of a .155".shirnresults.in aradius of about22.5 inches in thesheet when the stringers are placed 7 inches apart. I v

After the screws are tightened to prestress the sheet, the sheet is -tackwelded for example, at

8-inch intervals w'tha: .spotwelder such L s, for

am l ;eTe l tWZefi slee r et machine, and the screws removed. ;;'IThesheet-then resumes its. flat condition Twithoutfwarping, 'Jbuckling, nor .canningpresent. :The. ,spacesjbetween the tack- WeldSQcanLthen..bel filled. in .byY'additional spotling taking 'place.

1 welds one half incnapartwith'out further buck- A n alternativel and equally satisfactory means of pre-stressing the sheet alone prior to' spotwelding is shown in Figure 3. Here the entire sheet l is curved by being fastened, along with the stringers 4, to end plates 1 of wood, for example by wood. screws 5. A radius of about 22.5 inches has also been found satisfactory for .016 gauge 24 STAL in this embodiment. As before, the stringers are tackwelded to the skin, the end plates removed to permit the skin to flatten, and the space between welds filled by additional spot welds .5 inch apart. No buckling, warping, or canning will occur.

In the embodiment shown in Figure 1 some upward stress may be imposed upon stringers 4 if 3 they are long and held only at their ends by screws 5. This can be avoided by using several screws along the stringer length, if desired. In the embodiment of Figure 2, no stress at all is imparted to stringers 4, irrespective of their length, as the end plates hold the sheet in proper curvature.

In both cases, it will be seen that the skin sheet is prestressed by the formation of one or more column sections therein extending parallel to the line of spotwelds. The curvatures are within the elastic limit of the skin material and the skin returns to its original condition.

Although the operation of spotfastening by spotwelding has been applied successfully to the means and method of Figures 1 and 3, it should be distinctly understood that riveting instead of spotwelding may be employed, with equally good results. This success leads directly to a third embodiment of the invention shown in Figure 4, where the thin skin I is shown as being applied to a rigid frame I!) which, for example, may be a flap or similar airframe structure finished except for one skin surface. In this case the final skin surface must be blind riveted, which of course can be done by using various types of blind rivets such as, for example, explosive rivets.

In this case the skin is stressed close to the rivet line by the insertion of a. shim ll resting on the internal stringers l2. The line of rivets I3 is then made, and shim ll moved along the stringers to a point just beyond where the next line of rivets is to be inserted, and new rivets inserted. In this way the skin adjacent the rivets is always stressed, and the final skin is smooth and without buckles.

While spotwelds and rivets are the more usual types of spotfastening means used in aircraft construction, it is to be distinctly understood that the present invention is highly satisfactory to prevent canning when other spotfastening means are used such as, for example, belts or self-tapping screws, and I do not wish to be limited to any particular fastening means.

A number of variations from the means described above to illustrate the method of the present invention will be apparent to those skilled in the art without departing from the invention as set forth in the attached claims. The main feature of the present invention is that only the skin is prestressed adjacent the place where the spot fastening is made.

I claim:

1. For securing a thin metal skin at spaced points to spaced substantially parallel rigid frame means, a method comprising: temporarily securing the metal skin at spaced points to a spaced pair of said rigid frame means and to a rigid backing member; flexingsaid skin between said rigid frame means; permanently securing said skin to said rigid frame means at spaced points along the frame means; releasing said pair of frame means from the backing member to relieve the flexing in said skin; and repeating the steps set forth between consecutive pairs of frame means until the skin is secured to all said frame means.

2. A method according to claim 1, where the flexing is produced by positioning a narrow length of material between said backing member and said sheet parallel to the frame means, and thereafter tightening down the frame means on said rigid backing member.

3. A method according to claim 1, wherein the skin is permanently secured by riveting.

4. A method according to claim 1, wherein the skin is permanently secured by spotwelding.

5. For securing a thin metal skin at spaced points to spaced, substantially parallel and rela- -tively rigid frame means, a method comprising: temporarily securing the metal skin at spaced points to a spaced pair of said rigid frame means and to a rigid backing member with a narrow length of material positioned between said backing member and said sheet parallel to the frame means; tightening down the frame means on said rigid backing member whereby said skin is flexed between said frame means; tacking said skin to said rigid frame means at spaced points along the frame means; releasing said pair of frame means from the backing member to relieve the flexing in said skin; completing securement of the skin to the frame means at spaced points between the aforesaid spaced points, and repeating the steps set forth between consecutive pairs of frame means until the skin is secured to all saidframe means.

SETH P. JENKINS.

REFERENCES CITED The following references are of record in the file of this patent:

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